Transcript Atomic Absorption Spectroscopy

Atomic Absorption
Spectroscopy
We will cover two main techniques of
atomic absorption spectroscopy (AAS),
depending of the type atomizer. Two
atomization techniques are usually
used in AAS:
1
2
3
1. Flame Atomization
Flames are regarded as continuous atomizers
since samples are continuously introduced
and a constant or continuous signal is
obtained. Samples in solution form are
nebulized by one of the described
nebulization techniques discussed
previously. The most common nebulization
technique is the pneumatic nebulization.
Nebulized solutions are carried into a flame
where atomization takes place.
4
Several processes occur during
atomization including:
a. Nebulized samples are sprayed into a
flame as a spray of very fine droplets
b. Droplets will lose their solvent content
due to very high flame temperatures in
a process called desolvation and will
thus be converted into a solid aerosol.
c. The solid aerosol is volatilized to form
gaseous molecules
5
d. Gaseous molecules will then be
atomized and neutral atoms are
obtained which can be excited by
absorption of enough energy. If energy
is not enough for atomization, gaseous
molecules will not be atomized and we
may see molecular absorption or
emission
e. Atoms in the gaseous state can absorb
energy and are excited. If energy is too
much, we may observe ionization.
6
7
8
The different processes occurring in
flames are complicated and are not
closely controlled and predicted.
Therefore, it can be fairly stated that
the atomization process in flames may
be one of the important parameters
limiting the precision of the method. It
is therefore justified that we have a
closer look at flames and their
characteristics and the different
variables contributing to their
performance.
9
Types of Flames
Flames can be classified into several types
depending on fuel/oxidant used. For
example, the following table summarizes the
features of most familiar flames.
Therefore, it can be clearly seen that
significant variations in flame temperatures
can be obtained by changing the
composition of fuel and oxidant.
10
11
On the other hand, flames are only stable at
certain flow rates and thus the flow rate of
the gas is very important where at low flow
rates (less than the maximum burning
velocity) the flame propagates into the
burner body causing flashback and, in some
cases, an explosion. As the flow rate is
increased, the flame starts to rise above the
burner body. Best flames are obtained when
the flow rate of the gas is equal to the
maximum burning velocity. At this equity
ratio the flame is most stable. At higher
ratios, flames will reach a point where they
will no longer form and blow off the burner.
12
Flame Structure
Three well characterized regions can be identified in a
conventional flame. A lower region, close to the
burner tip, with blue luminescence. This region is
called the primary combustion zone which is
characterized by existence of some non atomized
species and presence of fuel species (C2 and CH,
etc.) that emit in the blue region of the
electromagnetic spectrum. The second well defined
region is called the interzonal region just above the
primary combustion zone. The interzonal region is
rich in free atoms and is the region of choice for
performing atomic spectroscopy.
13
It also contains the regions of highest
temperatures. The third region in the
flame is the outer region which is called
the secondary combustion region. It is
characterized by reformation of
molecules as the temperature at the
edges is much lower than the core.
These regions can be schematically
represented by the following
schematic:
14
15
Flame Absorption Profiles
We have seen that there are different temperature
profiles in a flame and temperature changes as the
distance from the burner tip is change
16
Atomic Absorption
Spectroscopy
Lecture 13
17
Flame Absorbance profiles
Since the temperature of a flame depends on the
position from its tip, it is necessary to concentrate
our work on one spot in a flame and preferably
adjust the height of the flame to get best signal. In
fact, not all elements require a specific height above
burner tip but rather each element has its own
requirements which largely reflect some of its
properties. For example, one can use higher
distances from the tip so that higher temperatures
are achieved to analyze for silver. This is possible
since silver will not be easily oxidized.
18
19
However, best results for the analysis of
chromium occur at lower heights (fuel rich
flames) since at higher heights oxygen from
atmosphere will force chromium to convert
to the oxide which will not be atomized at
flame temperatures. A third situation can be
observed for magnesium where increasing
the height above tip will increase the signal
due to increased atomization at higher
temperatures. However, at higher distances
the oxide starts to form leading to a decrease
in signal.
20
Flame Atomizers (Continuous
Atomizers)
There are several types of flame atomizers
available. The simplest is a turbulent flow
burner that is very similar to conventional
Bunsen burner. This type of burner suffers
from fluctuations in temperature since there
is no good mechanism for homogeneous
mixing of fuel and oxidant. The drop size of
nebulized sample is also inhomogeneous
which adds to fluctuations in signal. The
path length of radiation through the flame is
small which suggests a lower sensitivity of
the technique.
21
Turbulent flow burners are also susceptible to
flashback. These drawbacks were overcome
using the most widely used laminar flow
burner (also called premix burner) where
quite flames and long path length are
obtained. Flashback is avoided and very
homogeneous mixing between fuel, oxidant,
and droplets take place. Larger droplets are
excluded and directed to a waste container.
A schematic representation of the burner is
shown below:
22
23
24
25
26
27
Fuel and Oxidant Regulators
The adjustment of the fuel to oxidant
ratio and flow rate is undoubtedly very
crucial. Although stoichiometric ratios
are usually required, optimization is
necessary in order to get highest
signal. However, in the determination of
metals that form stable oxides, a flame
with excess fuel is preferred in order to
decrease oxide formation.
28
Performance Characteristics of
Flame Atomizers
Reproducibility of flame methods are usually
superior to other atomization techniques.
However, the residence time of an atom in a
flame is in the order of 10-4 s which is very
short. This is reflected in a lower sensitivity
of flame methods as compared to other
methods. Also, conventional flames with
reasonable burning velocities can produce
relatively low temperatures which make them
susceptible to interference from molecular
species.
29
2. Electrothermal Atomization
These have better sensitivities than flame
methods. The increased sensitivity can be
explained on the basis that a longer atom
residence time is achieved (can be more than
1 s) as well as atomization of the whole
sample in a very short time. As the name
implies, a few mL of the sample are injected
into the atomization chamber (a cylinder of
graphite coated with a film of pyrolytic
carbon) where the following processes take
place:
30
31
a. Evaporation: the solvent associated with the
sample is evaporated in a low temperature
(~120 oC) slow process (seconds)
b. Ashing: sample is ashed to burn organics
associated with the sample at moderate
temperatures (~600 oC, seconds)
c. Atomization: The current is rapidly increased
after ashing so that a temperature in the
range from 2000-3000 oC is obtained in less
than1 second.
32
Electrothermal Atomizers
(Discrete Atomizers)
The heart of the atomizer, beside efficient
heating elements and electronics, is a
cylindrical graphite tube opened from
both ends and has a central hole for
sample introduction. It was found that
porous graphite results in poor
reproducibility since some of the
analyzed materials will diffuse through
porous graphite and will thus lead to a
history effect.
33
34
Therefore, the cylindrical graphite is made
from a special type of nonporous high
quality graphite called pyrolytic graphite. The
length of the cylinder is 2-5 cm and it has
less than 1 cm diameter. When the tube is
fixed in place electrical contacts are
achieved which are water cooled. Two inert
gas streams (argon) flow at the external
surface and through the internal space of the
tube to prevent oxidation and clean the tube
after each measurement. Usually, samples
are analyzed in triplicates where three
consecutive reproducible signals are
required for each sample..
35
36
Atomic Absorption
Spectroscopy
Lecture 14
37
Performance Characteristics of
Electrothermal Atomizers
Electrothermal atomization is the technique of choice
in case of small sample size. Also, higher
sensitivities than flames are ordinarily obtained.
Unfortunately, the analysis time is in the few minutes
range and the relative precision is in the range of 510% as compared to 1% in flame methods. In
addition, the linear dynamic range is usually small (~
two orders of magnitude) which requires extra
sample manipulation. It may be also mentioned that
better experienced personnel can achieve the merits
of the technique.
38
Atomic Absorption
Instrumentation
Atomic absorption instruments consist
of a source of radiation, a
monochromator, a flame or
electrothermal atomizer in which
sample is introduced, and a transducer.
39
Radiation Sources
Although radiation in the UV-Vis region is required, we
can not use broad band sources. This is because
even the best monochromators can not provide a
bandwidth that is narrower than the atomic
absorption line. If the bandwidth of the incident
radiation is wider than the line width, measurement
will fail as absorption will be only a tiny fraction of a
large signal which is difficult to measure and will
result in very low sensitivities (figure a). Therefore,
line sources with bandwidths narrower than that of
the absorption lines must be used.
40
41
This can be achieved by using a lamp
producing the emission line of the
element of interest where analyte
atoms can absorb that line. Conditions
are established to get a narrower
emission line than the absorption line.
This can in fact be achieved by getting
an emission line of interest at the
following conditions:
42
1. Low temperatures: to decrease
Doppler broadening (which is easily
achievable since the temperature of the
source is always much less than the
temperature in flames).
2. Lower pressures: this will decrease
pressure broadening and will thus
produce a very narrow emission line.
43
This may suggest the need for a separate
lamp for each element which is
troublesome and inconvenient.
However, recent developments lead to
introduction of multielement lamps. In
this case, the lines from all elements
should not interfere and must be easily
resolved by the monochromator so
that, at a specific time, a single line of
one element is leaving the exit slit
44
Hollow Cathode Lamp (HCL)
This is the most common source in atomic absorption
spectroscopy. It is formed from a tungsten anode
and a cylindrical cathode the interior surface of
which is coated by the metal of interest. The two
electrodes are usually sealed in a glass tube with a
quartz window and filled with argon at low pressure
(1-5 torr). Ionization of the argon is forced by
application of about 300 V DC where positively
charged Ar+ heads rapidly towards the negatively
charged cathode causing sputtering. A portion of
sputtered atoms is excited and thus emit photons as
atoms relax to ground state. The cylindrical shape of
the cathode serves to concentrate the beam in a
limited region and enhances redeposition of
sputtered atoms at the hollow surface.
45
46
47
High potentials usually result in high
currents which, in turn, produce more
intense radiation. However, Doppler
broadening increases as a result. In
addition, the higher currents will
produce high proportion of unexcited
atoms that will absorb some of the
emission beam which is referred to as
self absorption (a lower intensity at the
center of the line is observed in this
case).
48
Electrodeless Discharge Lamps
(EDL)
An EDL is a sealed quartz tube containing a
few torr of an inert gas and a small quantity
of the metal of interest. Excitation of the
metal is achieved by a radiofrequency or a
microwave powered coil through ionization
of argon, due to high energetic
radiofrequency. Ionized argon will hit the
metal causing excitation of the atoms of the
metal of interest. The output power of the
EDL lamp is higher than the HCL lamp.
However, compared to HCL lamps, EDL
lamps are rarely used.
49
50
51
Emission in Flames
There can be significant amounts of emission
produced in flames due to presence of flame
constituents (molecular combustible
products) and sometimes impurities in the
burner head. This emitted radiation must be
removed for successful sensitive
determinations by AAS, otherwise a negative
error will always be observed. We can
visualize this effect by considering the
schematic below:
52
53
The detector will see the overall signal which is
the power of the transmitted beam (P) in
addition to the power of the emitted radiation
from flame (Pe). Therefore if we are
measuring absorbance, this will result in a
negative error as the detector will measure
what it appears as a high transmittance
signal (actually it is P + Pe). In case of
emission measurements, there will always be
a positive error since emission from flame is
an additive value to the actual sample
emission. It is therefore obvious that we
should get rid of this interference from
emission in flames.
54
Absorbance is defined as:
A = log (Po/P)
However, in absence of a sample the detector will
measure S1 , where:
S1 = Po + Pe
In presence of a sample, the detector will measure S2,
where:
S2 = P + Pe
Therefore A = log (Po + Pe)/(P + Pe)
At high absorbances, Pe may become much larger than
P and the absorbance will be a constant since both
Po and Pe are constants:
A = log (Po + Pe)/(Pe)
55
Source Modulation
It turned out that excluding the emission
signal from flames can easily be done
by an addition of a chopper to the
instrumental design. The chopper is a
motor driven device that has open and
solid (mirrors in some cases)
alternating regions as in the schematic:
56
57
The function of the chopper is to chop the light
leaving the source so that when the incident
beam hits the chopper at the solid surface,
the beam will be blocked and detector will
only read the emitted signal from the flame.
As the chopper rotates and the beam
emerges to the detector, the detector signal
will be the sum of the transmitted signal plus
that emitted from the flame. The signal
processor will be able to subtract the first
signal from the second one, thus excluding
the signal from emission in flames.
58
This can be represented by the following
equations:
Signal 1 (Blocked Beam) = Pe
Signal 2 (Transmitted Beam) = P + Pe
Overall Difference Signal = (P + Pe) - Pe =
P (Corrected Signal)
This correction method for background
emission in flames is called source
modulation.
59
The schematic of the AAS instrument with
source modulation correction can be
represented by the following schematic:
60
It should be recognized that addition of extra
components to an instrument will decrease
the signal to noise ratio and addition of a
moving component is usually regarded as a
disadvantage due to higher need for
maintenance.
Another procedure which can overcome the
emission from flames is to use a modulated
power supply that will give fluctuating
intensities at some frequency (say for
example pulsed radiation at a specific
frequency).
61
The emission from flames is a continuous
signal but that from the source is modulated.
Now if we use a high pass RC filter, only the
fluctuating signal will be measured as signal
while the DC signal will be considered zero
as it can not pass through the electronic
filter. The high pass RC filter is a device
which uses a resistor and a capacitor the
impedance of which is inversely proportional
to the frequency of the modulated signal.
Therefore, only high frequencies will have
low impedance and can pass through the
capacitor while signals of low frequencies
will suffer very high resistance and will not
be able to go through the capacitor.
62
AAS Instruments
Instruments in AAS can be regarded as
single or double beam instruments.
Single Beam Atomic Absorption
Spectrophotometers
A single beam instruments is the same
as the one described above (source
modulation section) or generally:
63
The term
“spectrophotometer”
implies that the
instrument uses a
dispersive
monochromator
(containing a prism
or a grating). Also,
the detector is a
photomultiplier tube
in most cases.
64
65
66
Double Beam Atomic Absorption
Spectrophotometers
In this type of instruments, the incident beam
is split into two beams of equal intensity by a
chopper with the solid surface being a
mirror. One of the beams will traverse the
sample in the atomizer while the other is
considered as a reference. Detector signals
will be consecutive readings of both the
reference and sample beams. The ratio of the
reference to the sample beams is recorded to
give the final signal.
67
A schematic representation of a double
beam instrument is shown below:
68
It should be emphasized here that in the
absence of sample, Pr is not equal to P
since the reference beam traverses
through air while the other beam
traverses through the flame. In flames,
particulates and molecular species
scatter and absorb a portion of incident
radiation, which results in a lower
intensity of the beam. To act as a real
double beam, The AA
spectrophotometer reference beam
should pass through a reference flame.
69
But even if we do that, there are no
guarantees that both beams will be of
equal intensities because it is almost
impossible to obtain exactly equivalent
flames. It is therefore important to
understand that the excellent features
of a double beam configuration are not
achievable in AAS instrumentation.
70
Atomic Absorption
Spectroscopy
Lecture 16
71
Interferences in Atomic
Absorption Spectroscopy
There are two major classes of interferences
which can be identified in atomic absorption
spectroscopy. The first class is related to
spectral properties of components other than
atomized analyte and is referred to as
spectral interferences. The other class of
interferences is related to the chemical
processes occurring in flames and
electrothermal atomizers and their effects on
signal. These are referred to as chemical
interferences and are usually more important
than spectral interferences.
72
Spectral Interferences
1. Spectral line Interference
Usually, interferences due to overlapping lines
is rare since atomic lines are very narrow.
However, even in cases of line interference, it
can be simply overcome by choosing to
perform the analysis using another line that
has no interference with other lines.
Therefore, line interference is seldom a
problem in atomic spectroscopy.
73
2. Scattering
Particulates from combustion products
and sample materials scatter radiation
that will result in positive analytical
error. The error from scattering can be
corrected for by making a blank
measurement. Scattering phenomenon
is most important when concentrated
solutions containing elements that
form refractory oxides (like Ti, Zr, and
W) are present in sample matrix.
74
Metal oxide particles with diameters
larger than the incident wavelength will
make scattering a real problem. In
addition, samples containing organic
materials or organic solvents can form
carbonaceous (especially in cases of
incomplete combustion) particles that
scatter radiation.
75
3. Broad Band Absorption
In cases where molecular species from
combustion products or sample matrix are
formed in flames or electrothermal
atomizers, a broad band spectrum will result
which will limit the sensitivity of the
technique. It should be indicated here that
spectral interferences by matrix products
are not widely encountered in flame
methods. Even if matrix effects are present
in flames, they can be largely overcome by
adjusting various experimental conditions
like fuel/oxidant ratio or temperature.
76
Another method for overcoming matrix
interferences is to use a much higher
concentration of interferent than that
initially present in sample material, in
both sample and standards (this
material is called a radiation buffer).
The contribution from sample matrix
will thus be insignificant.
Spectral interferences due to matrix are
severe in electrothermal methods and
must thus be corrected for.
77
Background Correction Methods
a.
The Two Line Correction Method
In this method, a reference line from the source
(from an impurity in cathode or any emission
line) is selected where this line should have
the following properties:
1.
Very close to analyte line
2.
Not absorbed by analyte
If such a line exists, since the reference line is
not absorbed by the analyte, its intensity
should remain constant throughout analysis.
78
79
However, if its intensity decreases, this
will be an indication of absorbance or
scattering by matrix species. The
decrease in signal of the reference line
is used to correct for the analyte line
intensity (by subtraction of the
absorbance of the reference from that
of the analyte). This method is very
simple but unfortunately it is not
always possible to locate a suitable
reference line.
80
b.
The Continuum Source
Method
This background correction method is the
most common method although, for reasons
to be discussed shortly, it has major
drawbacks and fails a lot. In this technique,
radiation from a deuterium lamp and a HCL
lamp alternately pass through the graphite
tube analyzer. It is essential to keep the slit
width of the monochromator sufficiently wide
in order to pass a wide bandwidth of the
deuterium lamp radiation.
81
In this case, the absorbance by analyte
atoms is negligible and absorbance can
be attributed to molecular species in
matrix. The absorbance of the beam
from the deuterium lamp is then
subtracted from the analyte beam
(HCL) and thus a background
correction is obtained.
82
83
84
Problems Associated with Background
Correction Using D2 Lamp
1.
The very hot medium inside the graphite
tube is inhomogeneous and thus signal is
dependent on the exact path a beam would
follow inside the tube. Therefore, exact
alignment of the D2 and HCL lamps should
be made.
2.
The radiant power of the D2 lamp in the
visible is insignificant which precludes the
use of the technique for analysis of analytes
in the visible region.
3.
Addition of an extra lamp and chopper
will decrease the signal to noise ratio.
85
c.
Background Correction Based on
Zeeman Effect
Zeeman has observed that when gaseous atoms (but
not molecules) are placed in a strong magnetic field
(~ 1 tesla), splitting of electronic energy levels takes
place. The simplest splitting of one energy level
results in three energy levels, one at a higher energy,
another at a lower energy (two s satellite lines) and
the third remains at the same energy as the level in
absence of the magnetic field (central p line).
Furthermore, the p line has twice the absorbance of
a s line and absorbs polarized light parallel to
direction of the magnetic field while the two s lines
absorb light perpendicular to magnetic field.
86
87
Light from a HCL lamp will pass through
a rotating polarizer that passes
polarized light parallel to external
magnetic field at one cycle and passes
light perpendicular to field in the other
cycle. The idea of background
correction using this method is to allow
light to traverse the sample in the
graphite furnace atomizer and record
the signal for both polarizer cycles
using the wavelength at the p line.
88
89
First cycle: light parallel to field; the p
line of the analyte absorbs in addition to
absorbance by matrix (molecular matrix
absorb both polarized light parallel or
perpendicular to field)
Signal a = Ap + AMatrix
b.
Second cycle: light perpendicular to field;
the p line of analyte will not absorb light
perpendicular to field and s lines will also
not affect absorbance at the p line
wavelength. Only matrix will absorb.
Signal b = AMatrix
a.
90
The overall signal is the difference of the
two signals = Ap
Therefore, excellent background
correction is achieved using the
Zeeman effect. This background
correction method results in good
correction and is usually one of the
best methods available.
91
Atomic Absorption
Spectroscopy
Lecture 17
92
Chemical Interferences
These are interferences resulting from
chemical processes occurring in flames and
electrothermal atomizers and affect the
absorption signal. To quantitatively assess
the effects of the different chemical
processes occurring in flames, one should
regard the burnt gases as behaving like a
solvent. This is necessary since our
knowledge of gaseous state reaction
equilibria is rather limited. Chemical
interferences include three major processes:
93
1. Formation of Compounds of
Low Volatility
Anionic species forming compounds of low
volatility are the most important. The
formation of low volatility species will result
in a negative error or at least will decrease
the sensitivity. For example, the absorption
signal of calcium will be decreased as higher
concentrations of sulfate or phosphate are
introduced. Cations forming combined
products with the analyte will also decrease
the signal obtained for the analyte. For
example aluminum forms a heat stable
compound with magnesium.
94
Elimination of Low Volatility
Compounds
1. Addition of a releasing agent: cations that
can replace the analyte (preferentially react
with the anion) are called releasing agents.
In this case the analyte is released from the
compound of low volatility and replaced by
the releaseing agent. Lanthanum or
strontium are good releasing agents in the
determination of calcium in presence of
phosphate or sulfate. Also, lanthanum or
strontium are good releasing agents in the
determination of magnesium in presence of
aluminum since both can replace
magnesium.
95
2. Addition of a protective agent: organic
ligands that form stable volatile
species with analytes are called
protective agents. An example is
EDTA and 8-hydroxyquinoline which
will form complexes with calcium
even in presence of sulfate and
phosphate or aluminum.
3. Use of higher temperature is the
simplest procedure to try if it is
possible
96
2. Dissociation Equilibria
Dissociation reactions occur in flames
where the outcome of the process is
desired to produce the atoms of
analyte. For example, metal oxides
and hydroxides will dissociate in
flames to produce the atoms as in the
equations
MO = M + O
M(OH)2 = M + 2 OH
97
Remember that we are not working in solution
to dissociate the compounds into ionic
species. In fact, not much is known about
equilibrium reactions in flames. It should
also be remembered that alkaline earth
oxides and hydroxides are relatively stable
and will definitely show characteristic broad
band spectra (more intense than line
spectra), except at very high temperatures.
The opposite behavior is observed fro alkali
metals oxides and hydroxides which are
instable even at lower flame temperatures
and thus produce line spectra.
98
An equilibrium can be established for the
dissociation of compounds containing atoms
other than oxygen, like NaCl where:
NaCl = Na + Cl
Now, if the signal from a solution of NaCl was
studied in presence of variable amounts of Cl
(from HCl, as an example), the signal will be
observed to decrease as the concentration of
Cl is increased; a behavior predicted by the
Le Chatelier principle in solutions.
99
The same phenomenon is observed
when a metal oxide is analyzed using a
fuel rich flame or a lean flame. Signal
will be increased in fuel rich flames
since the dissociation of metal oxides
is easier due to less oxygen while the
opposite takes place in lean flames
(oxygen rich).
100
A good example on dissociation equilibria can
be presented for the analysis of vanadium in
presence of aluminum and titanium, fuel rich
flames result in higher absorbance signal for
vanadium since the little oxygen present in
flames will be mainly captured by Al and Ti,
thus more V atoms are available. However, in
lean flames, excess oxygen is present and
thus vanadium will form the oxide and
addition of extra Ti and Al will not affect the
signal.
101
3. Ionization Equilibria
Ionization in fuel/air flames is very limited due
to relatively low temperatures. However, in
fuel/nitrous oxide or fuel/oxygen mixtures,
ionization is significant. Therefore, at
higher temperatures an important portion
of atoms can be converted to ions:
M = M+ + e
K = [M+][e]/[M]
102
Ionization in flames may explain the decrease
in absorption signal for alkali metals at very
high temperatures where as the temperature
is increased signal will increase till an extent
at some temperature where it starts to
decrease as temperature is further
increased; a consequence of ionization.
Therefore, usually lower flame temperatures
are used for determination of alkali metals. A
material that is added to samples in order to
produce large number of electrons is
referred to as an ionization suppressor, the
addition of which results in higher
sensitivities.
103
104
Practical Details in AAS
Sample Preparation
The most unfortunate requirement of AAS may
be the need for introduction of samples in
the solution form. This necessitates the
dissolution of the sample where in many
cases the procedure is lengthy and requires
very good experience. Care should be
particularly taken in order not to lose any
portion of the analyte and to make sure that
the reagents, acids, etc. used in the
dissolution and pretreatment of the sample
are free from analyte impurities.
105
I suggest that you follow exact
procedures for preparation of specific
samples for analysis by AAS. In some
cases where the sample can be
introduced directly to an electrothermal
atomizer without pretreatment (like
serum samples), definitely,
electrothermal atomizers will have an
obvious advantage over flame methods
which require nebulization.
106
Organic Solvents
1. Increased nebulization rate due to lower
surface tension of organic solvents which
produces smaller droplets as well as faster
evaporation of solvents in flames will result
in better sensitivities.
2. Immiscible organic solvents containing
organic ligands are used to extract metal
ions of interest and thus concentrate them
in a small volume (thus increasing
sensitivity) and excluding possible
interferences due to matrix components.
107
Calibration Curves
The absorbance of a solution is directly
proportional to its concentration but due to
the large number of variables in AAS, usually
this direct relationship may slightly deviate
from linearity. The standard procedure to do
is to construct a relation between the
absorbance and concentration for a series of
solutions of different concentrations. The
thus constructed graph is called a calibration
curve.
108
The unknown analyte absorbance is
found and the concentration is
calculated or located on the curve.
Neither interpolation nor extrapolation
is permitted to the calibration curve. A
sample can be diluted or the calibration
curve may be extended but always the
analyte absorbance should be within
the standard absorbance range
recorded. Usually, the concentration
axis has the ppm or ppb units.
109
Standard Addition method
Chemical and spectral interferences can be
partially or wholly overcome by the use of a
special technique of calibration called the
method of standard addition. In addition, the
use of this method provides better
correlations between standards and sample
results due to constant nebulization rates.
The method involves addition of the same
sample volume to a set of tubes or
containers.
110
Variable volumes of a standard are added
to the tube set followed by completion
to a specific volume. Now, all tubes
contain the same amount of sample but
different concentrations of analyte. A
plot is then made for the volume of
standard and absorbance. This plot will
have an intercept (b) with the y axis and
a slope equals m.
111
112
The concentration of the analyte can be
determined by the relation:
Cx = bCs/mVx
Where, Cx and Vx are concentration and
volume of analyte and Cs is the
concentration of standard.
One can only use two points to get the
analyte concentration using the
relation:
Cx = AxCsVs/(At –Ax)Vx
113
Detection Limits
Usually, atomic absorption based on
electrothermal atomization has better
sensitivities and detection limits than
methods based on flames. In general,
flame methods have detection limits in
the range from 1-20 ppm while
electrothermal methods have detection
limits in the range from 1-20 ppb.
114
This range can significantly change for
specific elements where not all
elements have the same detection
limits. For example, detection limits fro
mercury and magnesium using
electrothermal atomization are 100 and
0.02 ppb while the detection limits for
the same elements using flame
methods are 500 and 0.1 ppm,
respectively.
115
Accuracy
Flame methods are superior to electrothermal
methods in terms of accuracy. The relative
error in flame method can be less than 1%
while that for electrothermal method occurs
in the range from 5-10%. Also, electrothermal
methods are more susceptible to molecular
interferences from the matrix components.
Therefore, unless a good background
correction method is used, large errors can
be encountered in electrothermal methods
depending on the nature of sample analyzed.
116
Flame Photometry
The technique referred to as flame
photometry is a flame emission
technique. We introduce it here
because we will not be back to flame
methods in later chapters. The basics
of the technique are extremely simple
where a sample is nebulized into a
flame. Atomization occurs due to high
flame temperatures and also excitation
of easily excitable atoms can occur.
117
Emission of excited atoms is
proportional to concentration of
analyte. Flame emission is good for
such atoms that do not require high
temperatures for atomization and
excitation, like Na, K, Li, Ca, and Mg.
The instrument is very simple and
excludes the need for a source lamp.
The filter is exchangeable in order to
determine the analyte of interest and, in
most cases, a photomultiplier tube is
used as the detector.
118
119
120
121
Optical Atomic Spectra
We have briefly described in an introductory
chapter that atomic spectra are usually
line, rather than band, spectra due to
absence of vibrational and rotational
levels. The existence of quantized
electronic energy levels explains the origin
of the observed line spectra and exact
locations of possible lines
122